Drop-on-demand ink jet printing systems eject ink drops from printhead nozzles in response to pressure pulses generated within the printhead by either piezoelectric devices or thermal transducers, such as resistors. The printheads typically include a manifold that receives ink from an external ink supply and supplies ink to a plurality of pressure chambers. Each pressure chamber is fluidly coupled to the manifold by an inlet and by an outlet to a nozzle, which is an opening in an external surface of the printing system. On a side of the pressure chamber opposite the fluid path to the nozzle, a flexible diaphragm layer overlies the pressure chamber and the piezoelectric or thermal transducer is positioned over the diaphragm layer.
To eject an ink drop from a nozzle, an electrical firing signal activates the piezoelectric device or thermal transducer, which causes the piezoelectric or thermal transducer to bend the diaphragm layer into the pressure chamber. This movement urges ink out of the pressure chamber through the outlet to the nozzle where an ink drop is ejected. Each piezoelectric device or thermal transducer is individually addressable to enable the device or transducer to receive an electrical firing signal. Each structure comprised of a piezoelectric or thermal transducer, a diaphragm, a pressure chamber, and nozzle is commonly called an inkjet or jet. When the diaphragm rebounds to its original position, the ink volume in the pressure chamber is refilled by capillary action of the inlet from the manifold.
Inkjet printing technologies suffer from reliability concerns as one or more individual droplet ejecting nozzles may fail or malfunction in a printhead. In some cases, these failures are temporary because either a maintenance operation, such as a printhead purge, or the passage of time may enable the nozzles to recover and recommence operation. In some cases, however, the permanent failure of a single nozzle may force the replacement of an entire printhead. Most nozzle failures, temporary or permanent, are caused either by contamination, such as contaminants in ink or manufacturing debris, and external paper debris or by air bubbles either ingested or forming near the nozzles. Nozzle failures are generally proportional to print throughput, so the higher the printing volume, the more likely a nozzle will fail. The permanent failure of a single nozzle may require the replacement of a printhead because the resulting missing line or column of pixels can be visually objectionable. Even temporary nozzle failures may result in a portion or all of a print job being discarded. Many attempts in the inkjet industry have been made to compensate for missing nozzles without either having to replace the printhead or perform a maintenance operation before printing can resume. Of course, more robust systems capable of compensating for missing or malfunctioning nozzles without requiring printhead replacement or maintenance operations are desirable.